EP1540857A1 - Optical receiver having compensation for kerr effect phase noise - Google Patents
Optical receiver having compensation for kerr effect phase noiseInfo
- Publication number
- EP1540857A1 EP1540857A1 EP03794425A EP03794425A EP1540857A1 EP 1540857 A1 EP1540857 A1 EP 1540857A1 EP 03794425 A EP03794425 A EP 03794425A EP 03794425 A EP03794425 A EP 03794425A EP 1540857 A1 EP1540857 A1 EP 1540857A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- phase
- phase noise
- signal strength
- differential
- spans
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2543—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to fibre non-linearities, e.g. Kerr effect
Definitions
- the present invention relates generally to optical transmission systems and more particularly to apparatus and methods for reducing the effect of nonlinear phase noise caused by the interaction of optical amplifier noise and Kerr effect.
- each optical amplifier launches the same average signal power and each fiber span has the same length.
- « ⁇ ] 2 ⁇ 2 ⁇ 2 , where ⁇ 2 is the noise variance per span per dimension and E ⁇ .) denotes expectation.
- phase-modulated system uses binary phase shift keyed (BPSK) modulation where the binary "0" and “1 " are modulated onto a carrier by a carrier phase shift of 0 or ⁇ , respectively.
- BPSK binary phase shift keyed
- the statistical properties of « 2 are the same as those of n ⁇ .
- the signal arriving at the receiver is equal to E N by ignoring the fiber loss of the last fiber span and the optical amplifier required to compensate for it.
- Fig. 1 is an exemplary vector representation of a BPSK signal of "1" or "0" having constellation points with a phase of either 0 or ⁇ , respectively. Of course, any two phases that are separated by ⁇ may be used to represent “1" and "0".
- the vector representation is shown on a complex plane having an x-axis 12, aj ⁇ -axis 13, and an origin 14 at the intersection of the x and axes 12 and 13.
- the signal "1" is transmitted as represented by a vector 15.
- a vector 16 represents amplifier noise of a complex number of n ⁇ + ...+ n ⁇ in the absence of other effects.
- a vector 17 represents a received signal EM 15 with the effect of amplifier noise 16 without including an interaction of the amplifier noise and Kerr effect.
- Kerr effect phase noise The Kerr effect phase noise is equal to - ⁇ L ⁇ gP, where ⁇ is the nonlinear coefficient of the optical fiber, J e f f is the effective nonlinear length per span, and P is the optical intensity.
- the unit of electrical field is defined herein such that the optical intensity or power is equal to the absolute square of electric field.
- the nonlinear phase shift is usually represented by a negative phase shift in majority of the literature. The usage of positive phase shift does not change the physical meaning of the nonlinear phase shift. Herein, unless otherwise noted, the notation of negative phase shift is used.
- nonlinear phase noise is accumulated span after span due to the Kerr effect phase noise.
- the accumulated nonlinear phase noise is shown in an equation 1 below as nonlinear phase shift ⁇ L , an denoted as an angle 18:
- 2
- both the noise vector 16 and the nonlinear phase shift ⁇ N L 18 are accumulated span after span.
- the incremental nonlinear phase noise angle of the Mi span is - ⁇ eff
- the pattern 25 is plotted with 5000 noise simulation points.
- Each instance of the simulated complex field is graphed in the scattergram pattern 25 with real part as the x-axis 12 and imaginary part as the v-axis 13, and is represented as a single point according to the formula which is equal to with a mean nonlinear phase shift ⁇ NL > of about 1 radian taken out.
- the points of the scattergram pattern 25 are similar to the end of the vector of 20 where the whole figure is rotated by minus the angle of the mean nonlinear phase shift ⁇ NL>-
- the scattergram pattern 25 shows a random scattering of the simulation points due to contamination by the amplifier noise 16 and a shape having the appearance of a section of a helix due to the nonlinear phase shift of ⁇ NL- ⁇ NL >- Fig. 2 indicates that the rotation of the nonlinear phase shift of ⁇ NL- ⁇ NL 5 is correlated to the distance from the origin 14. In general, the farther the points are from the origin 14, the more the rotation about the origin 14.
- the BPSK signal is detected by determining whether the signal is in the right or left of the.y-axis 13, for "1" and "0", respectively.
- a conventional detector viewed as a polar detector determines whether the absolute value of the received phase ⁇ is less than ⁇ /2 for "1 " or greater than ⁇ /2 for "0".
- 60321419vl conventional detector viewed as a rectangular detector determines whether one of the quadrature components, cos( ⁇ ), is positive or negative for "1 " or "0", respectively.
- the intensity of the received signal need not be known in the detection process.
- the present invention provides a method and apparatus in an optical system using an estimate of signal strength for reducing the effect of nonlinear phase noise induced by the interaction of amplifier noise and Kerr effect.
- a receiver of the present invention includes a complex signal estimator and an intensity-scaled phase noise compensator.
- the complex signal estimator provides an estimate of the complex components of a received optical signal.
- the complex components may be in a polar form of phase and signal strength or a rectangular form of real and imaginary terms.
- the phase noise compensator uses signal strength for compensating for the nonlinear phase noise in the complex components in order to provide a phase noise compensated representation of the optical signal.
- the phase noise compensator includes a rotator for rotating the complex components by a scaled signal strength and a data detector using the rotated complex components for detecting modulation data.
- the phase noise compensator includes a curved region detector having curved regions. The curved regions have nonlinear decision boundaries based upon a scale factor. The curved region detector detects data by determining the region that encloses the complex components.
- the complex components and the signal strength may be differential between symbol periods.
- a receiver of the present invention includes a frequency estimator and an intensity-scaled phase noise compensator.
- the phase noise compensator shifts an estimated frequency by a scaled signal strength for compensating for nonlinear phase noise in a received optical signal.
- an intensity-scaled phase noise compensator of the present invention compensates for nonlinear phase noise phase by re-modulating an optical signal with a scaled signal strength.
- the signal strength scale factor of the present invention is determined based upon the number of spans in an optical transmission system.
- the present invention eliminates a portion of the Kerr effect phase noise that is due to the interaction of optical amplifier noise and Kerr effect.
- the variance of the residual nonlinear phase shift with correction is reduced to approximately one fourth of that of the variance of the received nonlinear phase shift without correction.
- the methods and apparatus of the present invention described herein can be used in an optical transmission system to approximately double the transmission distance by doubling the number of spans by allowing the transmission system to accumulate about twice the mean nonlinear phase if the noisy nonlinear phase shift is the dominant degradation.
- the lengths of the spans can be increased. For a fiber loss coefficient of 0.20 dB/km the lengths of the spans can be increased by about 15 kilometers by doubling the launched power per span.
- Fig. 1 is a vector diagram of the prior art illustrating the effect of optical amplifier noise and nonlinear Kerr effect phase shift in a phase-modulated system
- Fig. 2 is a scattergram of the prior art illustrating the nonlinear phase noise that is generated by the interaction of optical amplifier noise and Kerr effect in a phase-modulated system;
- Fig. 3 is a scattergram illustrating the effect of the compensation of the present invention for reducing the nonlinear phase noise of Fig. 2;
- Fig. 4 is a block diagram of a receiver of the present invention having a phase estimator and an intensity-scaled phase noise compensator;
- Fig. 5 is a block diagram of a receiver of the present invention having quadrature estimator and an intensity-scaled phase noise compensator;
- Fig. 6 is an illustration showing a nonlinear decision boundary of the present invention.
- Fig. 7 is a block diagram of a receiver of the present invention having a phase detector and an intensity-scaled nonlinear phase noise compensator using the nonlinear decision boundary of Fig. 6;
- Fig. 8 is a block diagram of a receiver of the present invention having a quadrature estimator and an intensity-scaled phase noise compensator using the nonlinear decision boundary of Fig. 6;
- Fig. 9 is a block diagram of a receiver of the present invention having a differential phase estimator and an intensity-scaled phase noise compensator;
- Fig. 10 is a block diagram of a receiver of the present invention having a differential quadrature estimator and an intensity-scaled phase noise compensator;
- Fig. 11 is a block diagram of a receiver of the present invention having a differential frequency estimator and an intensity-scaled phase noise compensator;
- Fig. 12 is an exemplary detailed block diagram a quadrature estimator of a receiver of the present invention.
- Fig. 13 is an exemplary detailed block diagram of a differential quadrature estimator of a receiver of the present invention.
- Fig. 14 is a block diagram of an intensity-scaled phase noise compensator of the present invention using a phase modulator.
- the present invention provides a method and apparatus for detecting phase modulated data of the received optical signal of an optical transmission system where the received phase is contaminated by the nonlinear phase noise induced by the interaction optical amplifier noise and Kerr effect of an optical fiber.
- the method and apparatus use the idea that the nonlinear phase shift 18 (Fig. 1) is correlated with the intensity of the received signal related to the lengths of vectors 17, 20 (Fig. 1).
- the present invention is based on the idea that the nonlinear phase shift is correlated with the received intensity. While both the nonlinear phase shift and the received intensity have noisy components, those noisy components are correlated and we use one to partially cancel another.
- 2 is the received signal intensity at the receiver.
- the optimal factor is ⁇ , is shown in an equation 2 below. The approximation in the equation 2 is valid for high signal-to-noise ratio.
- Fig. 3 shows a complex scattergram pattern 30 for the present invention.
- the pattern 30 is generated from 5000 simulation points for different noise combinations having nonlinear phase compensation according to the present invention.
- the mean nonlinear phase shift ⁇ NL > is about N ⁇ J eff
- the optimal scale factor for large number N of fiber spans is about 50% of the ratio of mean nonlinear phase shift ⁇ NL> and the mean received power
- the roughly round shape of the scattergram pattern 30 shows that the effect of the nonlinear phase shift 18 has been reduced.
- the parameter that can be used to characterize the effect of phase correction is to compare the variance of var( ⁇ NL + ocRjv) for residual nonlinear phase shift with the variance of var( ⁇ NL ) for the original nonlinear phase shift.
- the ratio of var( ⁇ ⁇ L + oJV)/var( ⁇ NL ) is about VA, giving a reduction of about 50% in terms of standard deviation.
- the mean nonlinear phase shift ⁇ NL> is approximately equal to N ⁇ J e ff
- the variance of nonlinear phase shift var( ⁇ L) is approximately equal to 4N ( ⁇ Z eff ⁇
- the variance of residual nonlinear phase shift var( ⁇ ⁇ L + oJV) is approximately equal to N 3 ( ⁇ Z eff ⁇ I E 0 ⁇ ) 2 13.
- SNR signal-to-noise ratio
- the optimal scale factor is equal to about 50% of the ratio of mean nonlinear phase shift and received power.
- the received signal is equal to the optical signal entering the last span. If the received signal after the last span is amplified using an optical amplifier with optical amplifier noise before the receiver, assume that the received power is equal to the launched power per span, the above discussion is still valid by changing N to N+l for the equation 2 with one more amplifier.
- the last span does not introduce nonlinear phase shift ⁇ ⁇ L- ⁇ quivalently, an optical amplifier before the receiver also does not introduce nonlinear phase shift ⁇ L -
- the importance of the current invention is that the use of the correction term of ⁇ Ry reduces the effect of nonlinear phase shift ⁇ L 18.
- the present invention can be implemented by using various methods and apparatus.
- complex components is used herein to mean a phase and a (positive) signal strength in a polar coordinate system or two signal (positive or negative) components in a rectangular coordinate system.
- the two components in the rectangular coordinate system are typically real and imaginary quadrature components or in-phase and quadrature-phase parts of the signal.
- the two components can be any two components that are not phase coincident.
- the "signal strength" of a signal may be in units of power or magnitude where magnitude is the positive square root of power. Information may be converted back and forth between units of power and magnitude with conventional circuit or software techniques.
- the complex components and the signal strength may represent a signal during a
- 60321419vl certain time or may represent a difference in the signal between two times as in a differential phase, differential quadrature components, or differential signal strength.
- Fig. 4 is a block diagram of a receiver of a preferred embodiment of the present invention referred to by a reference identifier 10 A.
- the receiver 10 A receives the optical signal ER 42 and provides a nonlinear compensated phase, denoted by 43, having a reduced level of unwanted nonlinear phase noise.
- the compensated phase 43 shown below as y ⁇ R + a N , is a phase noise compensated representation of the received optical signal ER 42.
- the receiver 10A includes a complex signal estimator 45 for receiving the optical signal E R 42 and an intensity-scaled phase noise compensator 47 for providing detected data 48.
- the complex signal estimator 45 includes a phase estimator 50 and a signal strength estimator 51.
- the phase estimator 50 estimates the actual phase ⁇ ⁇ of the optical signal E R
- the phase ⁇ ⁇ 55 is preferably the estimated difference between the phase ⁇ ⁇ of the optical signal E R 42 and a mean ⁇ ⁇ > of that phase.
- the signal strength estimator 51 estimates the actual power P N ⁇
- the power P N 56 is the estimated difference between the power P # of the optical signal E R 42 and a mean ⁇ P N > of that power.
- the phase noise compensator 47 includes a sealer 57, a phase rotator 58, and a polar data detector 59.
- Information for the scale factor ⁇ is stored by the sealer 57.
- the sealer 57 multiplies the estimated power P N 56 by the scale factor and passes the estimated phase ⁇ ⁇ and a scaled signal strength aP N to the phase rotator 58.
- the phase rotator 58 adds the scaled signal strength ⁇ P N to the phase ⁇ ⁇ in order to rotate the phase ⁇ ⁇ by a P N .
- the rotated phase ⁇ ⁇ + P N is the nonlinear compensated phase 43.
- the polar data detector 59 uses the nonlinear compensated phase ⁇ ⁇ + P N 43 for detecting the modulated data that is carried on the optical signal E R 42 and issuing the estimate as the detected data 48.
- the polar data detector 59 uses ⁇ /2 as a decision boundary.
- BPSK 60321419vl complicated phase-modulated system, or its variations combined with amplitude and/or frequency modulation
- the polar data detector 59 makes the decision whether "1" or "0" is transmitted by the criterion of whether the absolute value of the compensated phase 43 is less than ⁇ /2 for "1 " or greater than ⁇ /2 for "0".
- the polar detector 59 may require other information to make a decision if the original data is not only phase modulated, but also encoded in amplitude or frequency.
- the signal strength estimator 51 and the phase estimator 50 may be implemented as two separate blocks or combined into a single circuit or block.
- the nonlinear phase shift ⁇ NL 18 is described as a positive phase shift.
- the scale factor ⁇ is negative in the specific case in which the nonlinear phase shift is regarded as being a positive phase shift.
- Fig. 5 is a block diagram of a receiver of another preferred embodiment of the present invention referred to by a reference identifier 10B.
- the receiver 10B receives the optical signal E R 42 and provides nonlinear phase noise compensated quadrature components c and y c , denoted by 72, having a reduced level of unwanted nonlinear phase noise.
- the phase noise compensated quadrature components x c and y c 72 are a phase noise compensated representation of the received optical signal E ⁇ 42.
- the receiver 10B includes a complex signal estimator 73 for receiving the electric field of the optical signal E R 42 and an intensity-scaled phase noise compensator 74 for providing the detected data 48.
- the complex signal estimator 73 includes a quadrature estimator 75.
- the quadrature estimator 75 estimates of the actual quadrature components x R and y R of the optical signal ER 42 for providing estimated quadrature components x R and y R 80.
- the phase noise compensator 74 includes a signal strength estimator 76.
- the phase noise compensator 74 also includes a sealer 82, a quadrature rotator 83, and a rectangular data detector 85.
- Information for the scale factor ⁇ is stored by the sealer 82.
- the sealer 82 multiplies the power P N 81 by the scale factor a and passes the quadrature components x R and y R and a scaled signal strength aP N to the quadrature rotator 83.
- the quadrature rotator 83 rotates the quadrature estimates x R and y R by the scaled signal strength P N according to an equation 3, below, for providing the nonlinear phase noise compensated quadrature components c and y c 72.
- x c x R cos( ⁇ V ) - y R s (aP N )
- the rectangular data detector 85 uses the phase noise compensated quadrature components x c and y c 72 for providing the detected data 48 that is representative of the original encoded or modulated data that is carried on the optical signal ER 42.
- the rectangular data detector 85 makes the decision whether "1" or "0" is transmitted by the criterion whether one of the quadrature components x c of the compensated phase representation 72 is positive or negative, respectively.
- phase and amplitude modulation called quadrature-amplitude modulation (QAM) or the case of -ary phase-shift keying (MPSK)
- QAM quadrature-amplitude modulation
- MPSK -ary phase-shift keying
- both quadrature components of x c and y c 72 are required for the detector 85 to provide the detection data 48.
- the rectangular data detector 85 can be implemented as a traditional quadrature detector for the specific modulation technique.
- Fig. 6 shows an optimal nonlinear decision boundary 91 separating received signal- space into curved regions 92 and 94.
- the region 92 containing the signal point of "1” is the decision region to determine when "1 " is transmitted.
- the region 94 containing the signal point of "0” is the decision region to determine when "0” is transmitted.
- the curved regions 92 and 94 can be used for detecting the data 48 from the estimated quadrature components of ⁇ R and y R 80 (Fig. 5).
- the boundary 91 has a rotation angle for a mean nonlinear phase shift ⁇ NL>-
- the boundary 91 and the signal points "1 " and "0" are plotted without showing
- the decision region 92 for "1" is ⁇ R sin[ ⁇ ( 2 + y R 2 )]+y R cos[ ⁇ ( 2 + y )] ⁇ 0 and the decision region 94 for "0" is x R sin[ ⁇ ( 2 + yl)] + y R cos[ ⁇ (x 2 + y )] ⁇ 0.
- the decision regions 92 and 94 can also be expressed in polar coordinates. In polar coordinates the decision region 92 for
- T' is ⁇ ⁇ + ⁇ pj j ⁇ ⁇ /2 and the decision region 94 for "0" is ⁇ ⁇ + ⁇ pj j > ⁇ /2 ,
- the optimal shape of the boundary 91 depends upon the scale factor ⁇ .
- the boundary 91 is drawn for an optical transmission system using BPSK modulation and having a mean nonlinear phase shift ⁇ N L> of about 1 radian.
- mean nonlinear phase shift ⁇ NL > 0
- the boundary 91 becomes, in rectangular coordinates, the y-axis 13.
- the curved regions 92 and 94 touching at the decision boundary 91 can be implemented using a look-up table.
- the look-up table can be implemented for either complex quadrature components x R and y R or magnitude and phase components of p R and
- Fig. 7 is a block diagram of a receiver of the present invention referred to by a reference identifier IOC.
- the receiver IOC receives the optical signal ER 42 and uses one or more of the nonlinear decision boundary curves 91 (Fig. 6) with the estimated quadrature components x R and y R 80 for providing detected data 48 having a reduced level of unwanted nonlinear phase noise.
- the detected data 48 is a nonlinear phase noise compensated representation of the optical signal E R 42.
- the receiver 10C receives the optical signal E R 42 and uses one or more of the nonlinear decision boundary curves 91 (Fig. 6) with the estimated quadrature components x R and y R 80 for providing detected data 48 having a reduced level of unwanted nonlinear phase noise.
- the detected data 48 is a nonlinear phase noise compensated representation of the optical signal E R 42.
- the receiver 10C includes the quadrature estimator 75 and a phase noise compensator 108.
- the quadrature estimator 75 receives the optical signal E R 42 and provides the estimated quadrature components x R and y R 80.
- the phase noise compensator 108 includes a rectangular curved region detector 110 having stored information for the curved regions 92 and 94.
- the region detector 110 issues the detected data 48 according to which particular one of the curved regions 92 and 94 encloses the complex location of the quadrature components x R and y R 80.
- Fig. 8 is a block diagram of a receiver of the present invention referred to by a reference identifier 10D.
- the receiver 10D receives the optical signal E R 42 and uses one or more of the nonlinear decision boundary curves 91 (Fig. 6) with the estimated polar coordinates ⁇ ⁇ 55 and power P N 56 for providing detected data 48 having a reduced level of unwanted nonlinear phase noise.
- the detected data 48 is a nonlinear phase noise compensated representation of the optical signal ER 42.
- the receiver 10D includes a complex signal estimator 116 and a nonlinear phase noise compensator 118.
- the complex signal estimator 116 includes the phase estimator 50 for receiving the optical signal E R 42 and providing the estimated phase ⁇ ⁇ 55 and the signal strength estimator 51 for receiving the optical signal E R 42 and providing the estimated power P N 56.
- the phase noise compensator 118 includes a polar curved region detector 120 having stored information for the curved regions 92 and 94.
- the region detector 120 issues the detected data 48 according to which particular one of the curved regions 92 and 94 encloses the complex location of the power P N 56, corresponding to radius p 2 , and the phase ⁇ ⁇ 55.
- the region detectors 108 and 118 can be implemented with look-up tables. Several look-up tables may be included where each table corresponds to curved regions, analogous to the curved regions 92 and 94, for different scale factors ⁇ for different levels, respectively, of the mean nonlinear phase ⁇ N L >-
- Fig. 9 is a block diagram of a receiver of another preferred embodiment of the present invention referred to by a reference identifier 10 ⁇ .
- the receiver 10 ⁇ receives the optical signal E R 42 and provides a differential nonlinear compensated phase, denoted by 152, having a reduced level of unwanted nonlinear phase noise.
- the compensated phase 152 shown below as ⁇ ⁇ + a ⁇ P N , is a nonlinear phase noise compensated representation of the optical signal E R 42.
- the receiver 10 ⁇ is intended for a differential phase-shifted keyed (DPSK) system.
- DPSK differential phase-shifted keyed
- DPSK systems use the difference of the phase between two adjacent transmitted symbols to encode data.
- the data is encoded or modulated in ⁇ (t+7) - ⁇ (t) of the carrier, where / is time and T is the symbol interval.
- the nonlinear phase shift due to the interaction of optical amplifier noise and Kerr effect degrades performance.
- the data is decoded using ⁇ i?(t+T) - ⁇ ⁇ (t) without using the information from the received intensity.
- the difference in nonlinear phase shift fymit+ ) - ⁇ NL (t) is also correlated with the difference in received
- 60321419vl intensity P ⁇ (t+T) - Z C with the same correlation as that for BPSK system.
- a detector making decisions according to ⁇ / j(t+7) - ⁇ i?(t) + ⁇ [R/v(t+7) - P N )] can be used to reduce the effect of nonlinear phase shift.
- the optimal combining factor ⁇ for DPSK system is the same as that for BPSK system.
- the receiver 10E includes a complex signal estimator 153 for receiving the optical signal ER 42 and an intensity-scaled phase noise compensator 154 for providing the detected data 48.
- the complex signal estimator 153 includes a differential phase estimator 160 and a differential signal strength estimator 161.
- the phase noise compensator 154 includes the sealer 57, the phase rotator 58, and the polar data detector 59 operating on the differential phase ⁇ ⁇ 170 and the differential power 171 in the manner as described above in the detailed description accompanying Fig. 4 for the phase ⁇ ⁇ 55 and the power P N 56.
- Information for the scale factor is stored by the sealer 57.
- the sealer 57 multiplies the differential power A N 171 by the scale factor and passes the differential phase ⁇ ⁇ and a scaled differential power ⁇ AP N to the phase rotator 58.
- the phase rotator 58 adds the differential phase ⁇ ⁇ and the scaled differential power ⁇ P N in order to rotate the differential phase ⁇ ⁇ by the scaled differential signal strength cc AP N .
- the rotated differential phase ⁇ ⁇ + aAP N is the differential nonlinear compensated phase 152 of the received optical signal ⁇ R 42.
- the polar data detector 59 uses the differential nonlinear compensated phase ⁇ ⁇ + ⁇ ⁇ 152 for providing the detected data 48. For binary DPSK modulation, the polar data detector 59 uses ⁇ /2 as a decision boundary, i.e., the decision is made based on whether the absolute value of the differential nonlinear compensated phase is less than or greater than ⁇ /2.
- FIG. 10 is a block diagram of a receiver of another preferred embodiment of the present invention referred to by a reference identifier 10F.
- the receiver 10F receives the optical signal ER 42 and provides nonlinear phase noise compensated differential quadrature components ⁇ x c and Ay c , denoted by 182, having a reduced level of unwanted nonlinear phase noise.
- the compensated differential quadrature components Ax c and Ay c 182 are a nonlinear phase noise compensated representation of the optical signal ER 42.
- the receiver 10F includes a complex signal estimator 183 for receiving the optical signal E R 42 and an intensity-scaled nonlinear phase noise compensator 184 for providing the detected data 48.
- the complex signal estimator 183 includes a differential quadrature estimator 190 and the differential signal strength estimator 161.
- cos ⁇ and AyR
- sin ⁇ ⁇ of the optical signal E R 42 for providing differential quadrature component estimates Ax R and Ay R , denoted with a reference identifier 191, corresponding to the quadrature components of the differential phase of ⁇ ⁇ ⁇ ?
- the phase noise compensator 184 includes the sealer 82, the quadrature rotator 83, and the rectangular data detector 85.
- Information for the scale factor ⁇ is stored by the sealer 82.
- the sealer 82 multiplies the differential power AP N 111 by the scale factor ⁇ and passes the differential quadrature components Ax R and Ay R and a scaled differential power ⁇ AP N to the quadrature rotator 83.
- the quadrature rotator 83 rotates the differential quadrature estimates Ax R and Ay R according to an equation 4, below, for providing rotated differential quadrature component estimates A ⁇ c and ⁇ c as the compensated phase 182.
- Ax c Ax R cos( ⁇ P ⁇ , ) - Ay R sin( ⁇ P ⁇ r )
- the rectangular data detector 85 uses the rotated quadrature components of A ⁇ c and Ay c 182 for providing the detected data 48.
- Fig. 11 is a block diagram of a receiver of another preferred embodiment of the present invention referred to by a reference identifier 200.
- the receiver 200 receives the optical signal ER 42 and provides a nonlinear phase noise compensated frequency, denoted by 202, having a reduced level of unwanted nonlinear phase noise.
- the phase noise compensated frequency 202 shown below as ⁇ / + AP N IT , is a phase noise compensated representation of the optical signal E R 42.
- Some optical communication systems use frequency to encode data, a technique referred to as frequency-shifted keying (FSK).
- FSK frequency-shifted keying
- the data is encoded or modulated into the frequency that is equal to d ⁇ (t)/dt where d/dt denotes differentiation operation with respect to time.
- frequency estimators There are many different types of frequency estimators, most of them estimating the difference a carrier frequency and a frequency in a time interval between a time to and a time to + T, where the time J is a symbol time period.
- the nonlinear phase noise correction to the differential frequency is ⁇ [Pjv(t ⁇ +J) - R;v(* ⁇ )]/T-
- the optimal scale factor ⁇ for FSK is the scale factor ⁇ that is optimum for a BPSK system.
- the receiver 200 includes a differential frequency estimator 210; the differential signal strength estimator 161 for receiving the optical signal E R 42; and an intensity-scaled phase noise compensator 212 for providing the detected data 48.
- the frequency estimator 210 receives the optical signal ER 42 and estimates the differential frequency that encoded the transmitted data for providing the estimated differential frequency Af described above and denoted as 215.
- the differential signal strength estimator 161 receives the optical signal ER 42 and provides information for the estimated differential power AP N 171 as described above.
- the phase noise compensator 212 includes a sealer 218, a frequency shifter 222, and a frequency data detector 224.
- Information for the scale factor ⁇ is stored by the sealer 218.
- the sealer 218 multiplies the differential power AP N 171 by the scale factor and divides by
- the frequency shifter 222 adds the Af and ⁇ AP N IT provided by the sealer 218 in order to shift the frequency Af by the scaled differential power ⁇ AP N IT.
- the shifted frequency Af + ⁇ AP N IT is the nonlinear phase noise compensated frequency 202 of the received optical signal ER 42.
- the detector makes the decision whether "1" or "0" is transmitted by the criterion whether the shifted frequency 202 is positive or negative.
- the frequency data detector 224 uses the shifted frequency 202 for detecting the modulated data that is carried on the optical signal E R 42 and issuing the estimate as the detected data 48.
- Fig. 12 is a block diagram of a detailed exemplary implementation of the complex signal estimator 73 illustrated in Fig. 5 for the receiver 10B.
- the complex signal estimator 73 includes the quadrature estimator 75 and the signal strength estimator 76.
- the quadrature estimator 75 is implemented as a phase-locked homodyne receiver.
- a local oscillator 252 is a light source with an output E /0 253.
- the optical frequency and phase of local oscillator output 253 is the same as that of the phase and carrier frequency of the received signal E R 42.
- a homodyne receiver typically includes some means to match the polarizations of the received optical electric field E R 42 and the local oscillator optical electric field Eh 253, but this polarization-matching means is omitted from Fig. 12 for simplicity.
- a passive optical hybrid 254 combines the received optical signal E R 42 and the local oscillator field E /0 253 and gives four outputs, numbered 255, 256, 257, and 258, that are ⁇ /2 (90°) apart in phase.
- the four outputs are the zero-phase output of (E ⁇ +E /0 )/2 255, the ⁇ - phase output of (E ⁇ -E/ 0 )/2 256, the ⁇ /2-phase output of (E R +jE ⁇ 0 )/2 257, and the - ⁇ /2-phase output of (ER-J ⁇ I 0 )I2 258.
- photodetectors numbered 260, 261, 262, and 263, are used to convert the combined optical signal of optical hybrid outputs 255, 256, 257, and 258, respectively, to four photocurrents, numbered 264, 265, 266, and 267.
- the ⁇ /2- and - ⁇ /2-phase photocurrents, numbered 266 and 267, respectively, are fed into a
- Outputs 272 and 273 of the subtraction device 270 and 271, respectively, are estimations of the quadrature components of the received optical signal ER 42.
- both outputs 272 and 273 include wide-band electrical noise and are weak photocurrent, preamplifiers and low-pass filters, numbered 274 and 275, are used to amplify the signal and filter the high-frequency noise.
- An output 80a of preamplifier and low-pass filter 274 is the in-phase component estimate R .
- An output 80b of preamplifier and low- pass filter 275 is the quadrature-phase estimate y R .
- a loop filter 276 uses one of the quadrature components, in this case, the quadrature-phase quadrature component ⁇ 80b to give a control signal 277 to local oscillator 252. Using the control signal 277, the output 253 of local oscillator 252 is locked into the phase and frequency of the received optical signal ER 42.
- the output x R 80a and the output y R 80b are issued as the quadrature components 80 and provided to the signal strength estimator 76.
- the signal strength estimator 76 uses quadrature components x R and y R denoted by 80a and 80b to estimate the received power and gives the estimated power output 81 of P N - ⁇ + y R 2 .
- Two squaring devices, numbered 280 and 281, give the square of the two quadrature components of 2 , 282 and y 283, respectively.
- Fig. 13 is block diagram of a detailed exemplary implementation of the complex signal estimator 183 illustrated in Fig. 10 for the receiver 10F.
- the complex signal estimator 183 includes the differential quadrature estimator 190 and the differential signal strength estimator 161
- the differential quadrature estimator 190 is implemented as an interferometric receiver.
- the received signal E R 42 is split into two outputs 304 and 305 using a splitter 306.
- the splitting ratio of splitter 306 can be chosen such that the power of one output signal 304 is larger than that in another output signal 305.
- the signal 304 is passed to the differential quadrature estimator 190.
- the signal 305 is passed to the differential signal strength estimator 161.
- the differential quadrature component estimator 190 includes a splitter 310 for splitting the signal 304 into two equal-power outputs, numbered 311 and 312.
- the two splitter outputs of 311 and 312 are fed to two different interferometers, numbered 315 and 316, with different phase setting.
- Both interferometers 315 and 316 are Mach-Zehnder type interferometer that splits a signal into two branches, which propagate along paths of different path lengths and are recombined.
- the path length difference for the two interferometers 315 and 316 is approximately the symbol time T.
- the first interferometer 315 issues two output signals, numbered 320 and 321, that are proportional to and Ej ? (t)-Ei ?
- the second interferometer 316 issues two output signals, numbered 322 and 323, that are proportional to E R (t)+jE R (t-T) and E R (t)-jE R (t-T) as the ⁇ /2- and - ⁇ /2-phase output, respectively.
- Four photo-detectors numbered 325, 326, 327, and 328, are used to convert the optical signals 320, 321, 322, and 323, respectively, to four photocurrents, numbered 330, 331, 332, 333, respectively.
- the zero- and ⁇ -phase photocurrents 330 and 331 are fed into a subtraction device 340 to get their difference as 341.
- the ⁇ /2- and - ⁇ /2-phase photocurrents 332 and 333 are fed into a subtraction device 342 to get their difference as 343.
- the outputs 341 and 343 of the subtraction devices 340 and 342, respectively, are estimations of the differential quadrature components.
- preamplifiers and low-pass filters numbered 345 and 346, are used to amplify the signal and filter the high-frequency noise.
- An output 191a of preamplifier and low-pass filter 345 is the in-phase differential estimate Ax R .
- the output 191b of preamplifier and low-pass filter 346 is the quadrature-phase estimate Ay R .
- the outputs 191a and 191b taken together comprise the differential quadrature components denoted as 191 in Fig. 10.
- a controller 350 uses both components Ax R and Ay R 191a and 191b for providing a control signal 351 to interferometer 315 and another control signal 352 to interferometer 316.
- the interferometers 315 and 316 use the control signals 351 and 352 to maintain the correct phase relationship.
- the differential signal strength estimator 161 includes the signal strength estimator
- the signal strength estimator 51 includes a photo-detector 360 to estimate the intensity of the received signal ER 42.
- a photocurrent 361 from the photo-detector 360 is amplified and filtered using a preamplifier and low-pass filter 362.
- 60321419vl low-pass filter 362 is an estimation of the intensity of the received signal ER 42.
- the estimated power 363 is delayed by a symbol time of T by a delay device 364.
- a subtraction device 366 subtracts the delayed estimated power 365 from the current estimated power 363 for providing the differential power 171.
- Fig. 14 is a block diagram an intensity-scaled phase noise compensator of the present invention referred to by a reference identifier 400.
- the compensator 400 receives an optical signal E R 42 and provides a nonlinear phase noise compensated output optical signal, denoted by 406, having a reduced level of unwanted nonlinear phase noise.
- the output optical signal is a nonlinear phase noise compensated representation of the received optical signal E R 42.
- the received optical signal E R 42 is split into first and second input signals 401 and 402 using a splitter 4.03.
- the splitting ratio of splitter 403 can be chosen such that the power of the first input signal 401 is larger than the power in the second input signal 402.
- the first input signal 401 is fed into an optical phase modulator 405 to give the output optical signal 406.
- the second input signal 402 is passed to an optical intensity sealer 408.
- the optical intensity sealer 408 includes the signal strength estimator 51, a multiplier 410, information for the scale factor , and a modulator driver 412.
- the signal strength estimator 51 preferably using a photodetector, a preamplifier and a low pass filter, estimates the power in the second input signal 402 for providing an estimated power 413.
- the multiplier 414 multiplies the estimated power 413 by the scale factor ⁇ for providing a correction voltage 416.
- the correction voltage 416 is amplified by the modulator driver 412 to give a nonlinear phase noise compensation drive signal 418.
- the drive signal 418 drives the phase modulator 405 to modulate the first input signal 401 for providing the output optical signal 406.
- the output optical signal 406 is a nonlinear phase noise compensated representation of the received optical signal E R 42.
- the output optical signal 406 of the phase modulator 405 can be fed to a receiver for data detection or continue its propagation through spans of optical fiber.
- the phase modulator 405 modulates phase at the frequency of the optical signal E R 42.
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Abstract
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US233917 | 1994-04-28 | ||
US10/233,917 US6915084B2 (en) | 2002-09-03 | 2002-09-03 | Optical compensator having compensation for Kerr effect phase noise |
PCT/US2003/018867 WO2004023680A1 (en) | 2002-09-03 | 2003-06-13 | Optical receiver having compensation for kerr effect phase noise |
Publications (2)
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EP1540857A1 true EP1540857A1 (en) | 2005-06-15 |
EP1540857A4 EP1540857A4 (en) | 2007-04-18 |
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EP03794425A Withdrawn EP1540857A4 (en) | 2002-09-03 | 2003-06-13 | Optical receiver having compensation for kerr effect phase noise |
Country Status (4)
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US (2) | US6915084B2 (en) |
EP (1) | EP1540857A4 (en) |
AU (1) | AU2003248697A1 (en) |
WO (1) | WO2004023680A1 (en) |
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CN109462435B (en) * | 2018-12-25 | 2020-06-09 | 武汉邮电科学研究院有限公司 | Nonlinear effect analysis model and method in system of optical fiber communication system |
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US7013089B1 (en) | 2006-03-14 |
WO2004023680A1 (en) | 2004-03-18 |
AU2003248697A1 (en) | 2004-03-29 |
EP1540857A4 (en) | 2007-04-18 |
US6915084B2 (en) | 2005-07-05 |
US20040042802A1 (en) | 2004-03-04 |
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